Telephone system



March 1 1932.- J. WICKS 1,847,154

TELEPHONE SYSTEM Filed April 30, 1950 2 Sheets-Sheet 1 n LL R g czrb my R a;- g 2 \ZT IT [T T1 M I WW I a a -51 i 1 u i u MAIN EXCHANGE Jul-m Hicks March 1, 1932. J. WICKS TELEPHONE SYSTEM Filed April 30, 1930 2 Sheets-Sheet 2 ITLUETLIEJP John wicks Patented Mar. 1, 1932 UETED STATES PATENT oFFicE JOHN HICKS, 01E OAK PARK, ILLINOIS, ASSIGNOR, BY MESNE ASSIGNMENTS, TO A- SOCIATED ELECTRIC LABORATORIES, INC, OF CHICAGO, ILLINOIS, A CORPORATION OF DELAWARE TELEPHONE SYSTEM Application filed April 30,

-. changes are at difierent potentials and-that the difference in ground potential between any swo exchanges 'may vary suddenly due to the changes in load conditions on nearby power systems, electrical railways, etc. This difference in ground potential between two exchanges may seriouslyinterfere with the proper operation of the exchange equipment on interoffice calls between such exchanges nis is especially true in systems such as disclosed in United States Patent No. 1,570,022,

issued January 19, 1926, to John Wicks, in which supervision and release are controlled by changing from ground to battery and vice versa at each end of the dial leg of a composite trunk. An arrangement for compensating for the difference in ground potential between two exchanges is disclosed in United States Patent No. 1,26?,O31, issued May 21, 1928, to Bernard B. Willis. The arrangement disclosed therein has the disadvantage that a separate conductor or compensating circuit is required between exchanges for each trunk. Furthermore, the arrangement compensates for difference in ground potential only in so far as the operation of the line relays at each end of the trunk is effected. The main object of the present invention, therefore, is the provision of a device which i will automatically compensate for a varying 1930. Serial No. 448,698.

bar.

According to another feature of the invention, only one compensating circuit is required between any two exchanges irrespective of the number of trunks connecting such exchanges, and this circuit need only comprise a single conductor.

Otherfeatures of the invention relating to the detail circuit arrangement will be apparent from the following specification.

A complete understanding of the invention may be had from the following explanation of the same when read in connection with the accompanying drawings comprising Figs. 1, 2 and 3, of which Figs. 1 and 2, when Fig. 2 is placed to the right of Fig. 1, show by means of the usual schematic diagram a preferred embodiment of the invention, and Fig. 3 shows the operating mechanism of the switch S shown in Fig. 2. a

Fig. 1 of the drawings showsthe wellknown composited trunk arrangement comprising the two physical trunks X and Y connecting a main exchange, indicated at the left of the drawings, and a sub-exchange indicated at the right of the drawings. This composite trunk arrangement is the same as that disclosed in the above referred to patent to John Wicks; The repeater E and selectorrepeater F shown in skeleton form on the drawings are the same as the switches E and F disclosed in the above-mentioned patent. Only a portion of these circuits have been shown to indicate the manner in which supervision and release are controlled over the dial leg of the trunk. The reference characters designating the relays and contacts of these circuits are the same as those used in the above-mentioned patent. The main exchange battery is indicated at A and the exchange bus-bars at 150 and 151.

Fig. 2' of the drawings shows the equipment in the sub-exchange for automatically compensating for a varying difl erence in ground potential between the two exchanges connected by the composited trunks shown in Fig. 1. This consists of a polarized relay 1, a two-direction stepping switch S, and the relays which control the insertion of the auxiliary cells in the leads extending from the main bus-bars of the exchange to the auxiliary bus-bars. Relays 27 to 34, inclusive, shown in the upper portion of the drawings control the insertion of cells in the positive battery lead while the relays 87 to 94, inclusive, shown in the lower portion of the drawings are used to insert the auxiliary cells in the negative battery lead. The regular 48 volt exchange battery is indicated at B. Busbars 63 and 62 are the main battery and ground bus-bars to which the battery connections of the exchange equipment are made. The auxiliary bus-bars 142 and 143 are provided to supply ground and battery to relays, such as 127 and 60, of all interofiice repeaters.

Fig. 3 of the drawings shows the operating mechanism of the two-direction stepping switch S of Fig. 2. This consists of a shaft 150 upon which are mounted the two stepping ratchet wheels 144 and 146, the holding ratchet wheel 148, and the wiper 16. The teeth of the ratchet 144 are adapted to be engaged by the driving pawl 145 when magnet 14' is energized, thereby stepping the swi'tch'in counter-clockwise direction. The teeth of'ratchet 146 are engaged by driving pawl 147 ofniagnet 15 to step the switch in a clockwise direction. A spring detent 149 is arranged to engage the teeth of the ratchet wheel 148 andto hold the switch in the position to which it is operatedbut allowing it to be'operated in either direction at any time.

Referring in detail to Fig. 1 of the drawings, it will be seen that the dial leg of the phantom trunk terminating at one end in repeater E and at the other end in selectorre-peater F'is normally connected to ground at each end of the trunk. This dial leg coinrises conductor 51 in the main exchange the k lower conductor of the physical trunk X, and conductor55 in the sub-exchange, and is connected to ground through the winding of relay 130 in repeater E, and to the auxiliary ground bus-bar through the winding of relay 60 of'selector-repeater F by way of conductor 17' When the repeater E is seized for an outgoingcall, relay 40 operates and closes a circuit for relay 41. Relay 41 upon operating transfers the conductor 51 from the winding ofrelay 130 to the winding of relay 45 and battery. Relay 45 of repeater E and relay 6O of selector-repeater F are thereby operated in series over the-dial leg. Tmpulses are repeated at contact of relay 40. Relay 6O of'seleetor-repeater Fin the sub-exchange repeats the impulses to the succeeding switches and'the connection is established in the usual manner. When the called subscriber answers the connector switch (not shown) reverses battery over the trunk thereby causing the operation of relay 77 of selectorrrepeater F. Relay 7 7 transfers the conductor from the Winding of relay and ground to the winding of relay 127 and battery. Since both ends of the dial leg are now connected to battery, relay 45 of repeater E deenergizes and brings about the reversal of current to the called line in the well known manner.

At the end of the conversation, and assuming that the calling subscriber hangs up first, line relay 40 of repeater E deenergizes and opens the circuit of relay 41. The deenergization of relay 41 transfers conductor 51 from the winding vof relay 45 and battery to the winding of relay 130 and ground. Relay 130 of repeater E and 127 of selector-repeater F energize in series over the dial leg of the trunk. The energization of relay 127 brings about the release of the selector-repeater F in the sub-exchange and the deenergization of relay 77. Relay 77 upon deenergizing transfers'the conductor 55 from the winding of relay 127'and battery to the winding of relay 60 and ground. Since both ends of the dial leg of the trunk are now connected to ground,

relay 130 of repeater E deenergizes and brings,

about the restoration of the switch train in the calling exchange.

When the called subscriber hangs up the connector used in the establishment of the connection will be released. In case the called subscriber hangs up first, the connector again reverses the battery connections over the trunk and relay 77 deenergizes. Conductor 55' is thereby transferred from the winding of relay 127 and battery to the winding of.

relay 60 and ground. Relay 45 of repeater E thereupon operates over the dial leg of the trunk and in a manner not shown causes the repeater E to again reverse the battery connection to the calling line to normal.

hen the calling subscriber now hangs up his receiver, line relay 40 of repeater E falls back and opens the circuit of relay 41. Relay 41 upon releasing transfers conductor 51 to the winding of relay 180 and ground, thereby bringing about the release of relay 60 of selector-repeater F and the release of the automatic switches used in the establishment of the connection.

From the above brief explanation of the operations of rep-eater E and selector-repeater F, it is at once apparent that the exchange ground in the two exchanges should be at substantially the same potential in order to insure a proper working of the equipment on interoflice calls. If the difference in ground potential between the two exchanges were not corrected, the voltage on the relays at each end of the trunk might be only about half of the regular exchange voltage when one end of the dial leg is connected to battery and the other to ground. Likewise when both ends of the dial leg are connected to ground through the windings of relays 130 and 60, the difference in ground potential might cause these relays to operate. It is, therefore, readily seen that in order to secure a proper operation of the equipment on interexchange calls, Si

exchange. The other end of the leg is connected to the windings of a polarized relay in the sub-exchange. The other terminal of the polarized relay winding is connected to-an auxiliary ground bus-bar which is used to furnish ground to relays, such 60, of all interofhce repeaters. Thus with the ground potential of the two exchanges substantially at the same potential, a circuit may be traced from ground, conductor 20, conductor 21 of the trunk Y, conductor 19, windings of the polarized relay 1 in multiple, back contact 13 of relay 11, bus-bar 142, conductor 141, normally-closed contact 71 of relay 31, normallyclosed contact 38 of relay 27, conductor 140, bus-bar 62, to the positive pole of battery B which is connected to exchange ground. As

long as the difference in ground potential between the two exchanges does not exceed 5 volts the polarized relay 1 will not operate in this circuit.

It will now be assumed that the ground potential in the main exchange rises 6 or 7 volts on account of changes in load conditions on nearby power systems. The difilerence in potential between the terminals of the windings of polarized relay 1 will then cause the relay to operate to close its contact 3, thereby completing an obvious circuit for relay 5. Relay 5 operates and at contact 8 completes a circuit for the stepping magnet 15 of the switch S. The switch S thereupon takes one step in a clockwise direction causing its wiper 16 to engage bank contact 23. Belay 5 in operating also closes its contact 9, thereby completing a circuit for relay 10.

Relay 10 operates and at contact 12 completes a circuit for the slow-to-operate relay 11. After an interval relay 11 operates and opens contact 13, thereby opening the circuit of the polarized relay 1 and allowing it to release. Relay 1 in releasing opens the circuit of relay 5 which releases and in turn opens the circuit to the stepping magnet 15 and at contact 9 opens the circuit of relay 10. Re-

lay 1O releases and at contact 12 opens the lay 10, and relay 11 will allow the insertion of auxiliary cells in the leads to the auxiliary bus-bars to compensate for the difference in ground potential, as will now be explained.

When Wiper 16 engages bank contact 23, a

circuit is closed from ground, wiper 16 of switch S, bank contact 23, through the windings of relay 27 and relay 87 in multiple. Relays 27 and 87 operate in this multiple circuit. The operation of relay 27 closes con- 2 tacts 36, 37 and 39 and opens contact 38, thereby inserting the auxiliary cell 134 between the main and auxiliary ground busbars in the following circuit: from the positive pole of the battery B and ground, busbar 62, conductor 140, contact 37, normallyclosed contact of relay 28, positive pole of the cell 134, negative pole of cell 134, contact 39 of relay 27, conductor 141, to the auxiliary ground bus-bar 142. The resistance 61 is provided so that the cell 134 will not be short circuited when contacts 36 and39 are'closed and before the contact 38 is opened. Thus it is seen that the positive pole of cell 134 has been connected to the positive pole of the battery B, thereby raising the potential of the positive bus-bar 142 by the voltage of cell 134.

The operation of relay 87, which occurs simultaneously with the operation of relay 27, closes contacts 96, 97 and 99 and opens contact 98, thereby inserting the cell 138 in the lead extending to the bus-bar 143 in the following circuit: the negative pole of battery B, bus-bar 63, conductor 139, contact 97 of relay 87, normally-closed contact 105 of relay 88, positive pole of cell 138, negative pole ofcell 138, contact 99 of relay 87, and conductor 67 to the negative bus-bar 143. The resistance is provided to prevent the sliort-cii'cuiting of cell 138 after contacts 96 and 99 have been closed and before contact 98 has been opened. Thus the cell 138 has been inserted in the negative lead to the bus-bar 143 with its positive pole connected to the negative pole of the battery B, 5;

thereby raising the potential of the auxiliary ba-tterybus-bar by an amount equal to the voltage of cell 138. The insertion of the auxiliary cells in the battery and ground leads to the repeater bus-bars occurs during the time interval required for the operation and release of the controlling relays 5, 10 and 11. Therefore, when the relay 11 releases and again connects the winding of the polarized relay 1 to the ground bus-bar 142, the potential of this bus-bar has been adjusted so that it is now at substantially the same potential as the exchange ground of the main exchange which is connected to the other ter- 5 minal of the polarized relay over the compensating leg 21of the composite trunk. The polarized relay 1, therefore, does not again operate until the exchange ground in either exchange has varied enough so that the difference in ground potential between the two exchanges again exceeds 5'volts.

It will now be assumed that the ground potential in the, sub-exchange in which the compensating equipment is located 1 suddenly rises 15 to 20 volts. As soon as the-difference in-ground potent-ialzexceeds 5 volts-the polarized relay 1 will operate and close its contact 2, since the potential across its-terminals-is now. in the opposlte d1recti0n,thereby'completlng an obvious circuit for relay 4.

Relay 4 operates and at contact 6 completes the circuit of stepping magnet 14 which energizes and causes the switch S to take one step in a counter-clockwise direction,,thereby advancingthe wiper 16 into engagement with the con-tact 22. Relay 4 also'closes its contact 7, thereby completing a circuit for relay 10 which in turn operates and completes the circuit. of. relay 11'. Relay 11 after an interval, operates and opens its contact 13, thereby opening the circuit of the polarized relay 1 and allowing it to release.

In the meantime, as soon as wiper 16 is stepped out of engagement with bank. COX-1'."

tact, 23, the circuitsfof relays 27 and 87 are opened and these relays release. The release of relay 27 opens contacts 37- and 39 thereby opening the connection to the auxiliary cell, 134, and closes lts contact 38, thereby again.-

completing the normal connection from the posltive pole of battery Band ground to the positive auxiliary bus-bar 142 as follows? positive pole of battery Band ground, bus bar 62, conductor 149,.normally-closed con-- tact 38 of relay 27, normally-closed contact" 71. of relay-31, conductor 141,.to the ground. busebar 142. Relay 87 inreleasingopensits contacts 97. and 99, thereby opening the connection-to the auxiliary cell 138, and closes its contact 98, thereby again completing the normal connection from the negative pole of battery B to the negative auxiliarybus-bar 143 as follows: negative pole of battery B,

bus-bar 63,. conductor 139, normally-closedcontact 98 of relay 87, normally-closed con tact 1090f relay 91, conductor 67, to the negative battery bus-bar 143.

When relay 11 releases following the release, of the polarized relay 1 and the relays 5 and 10, the closing of contact 13 again completes the circuit of the polarized relay 1.

The potential on each of the bus-bars 142 and 143 has been lowered approximately 5 volts cuit is closed from ground, wiper 16, bank contact 24, and through the windings of relays 31 and 91 in multiple to battery. Relay 31 operates and closes itscontacts 69, and

73 and opens contact 71, thereby inserting ductor 141, to the auxiliary positive bus-bar 142. Thus it is seen that the negative pole of the auxiliary cell 131 is connected to the positive pole of the battery B, thereby lowering the potential of the positive bus-bar 142 by an amount equal to the voltage of cell 131.

Relay 91 upon operating in the above traced circuit, closes its contacts 108, 110 and 111 andopens contact 109, thereby inserting the auxiliary cell 135 in the negative lead to the bus-bar 143 in the following circuit: from the negative pole of battery B, bus-bar 63, conductor 139, normally-closed contact 98 of relay 87, contact 110 of relay 91, normallyclosed contact 117 of relay 92, negative pole of the auxiliary cell 135, positive pole of cell.

135, contact 111 of relay 91, conductor 67, to

the auxiliarynegative battery bus-bar 143..

Thus the negative pole of cell 135 is connected to the negative pole of the battery B, thereby lowering the potential of the negative bus-bar 143 by an amount equal to the voltage of cell 135.

lVhen the relays 4, 10 and 11 have restored to normal upon the release of the polarized relay 1 when the contact 13 is opened, contact- 13 is again closed and the circuit of the polarized relay 1 is completed. The operation of relays 31 and 91 lowered the potential of the bus-bar 142 and 143 by approximately 5 volts but the difl'erence in potential on the terminals of the polarized relay 1 still exceeds 5 volts and the relay is again operated to close its contact 2. In the same manner as previously described this brings about the energization of stepping magnet 14 and the advancing of the switch wiper 16 into engagement with bank contact 25.

As soon as the switch wiper engages bank contact 25, a circuit is closed as follows: from ground, switch wiper 16, bank contact 25, and through the windings of relays 32 and 92 in multiple to battery. Relay 32 operates in this circuit and at contact 75 closes a locking circuit for relay 31 by way of contact 68 be fore relay 31, which is slow-to-release due to the resistance shunted around its winding, has had time to release. Relay 32 also closes its contacts 76 and 78 and opens its contact- 7 9, thereby inserting the auxiliary cell 132 in the positive lead extending to the bus-bar 142 in series with the auxiliary cell 131. The circuit is as follows: from the positive pole of battery B and ground, bus-bar 52, conduetor 140, normally-closed contact'38 of relay 27', contact 7 O of relay 31, contact 7 8 of relay 32, normally-closed contact 85 of relay 33, negative pole of cell 132, cells 132 and 131 in series, and from the positive pole of cell 131, contact 7 3 of relay 31, conductor 141 to positive bus-bar 142. Thus the potential of the positive bus-bar has been lowered below the normal ground potential by an amount equal to the voltage of the two cells 131 and 132.

Relay 92, upon operating in the above traced circuit, at contact 114 closes a locking circuit for relay 91 by way of contact 107 before relay 91, which is made slow-to-release by the resistance in shunt of its winding, has had time to deenergize. Relay 92 also closes its contacts 115 and 116 and opens its contact 117, thereby inserting the auxiliary cell 136 in series with cell 135 in the negative lead extending to the bus-bar 143. This circuit may be traced as follows: from the negative pole of battery B, bus-bar 63, conductor 139, normally-closed contact 98 of relay 87, contact 110 of relay 91, contact 116 of relay 92, normally-closed contact 123 of relay 93, negative pole of the auxiliary cell 136, cells 136 and 135 in series, positive pole of cell 135, contact 111 of relay 91, and conductor 67 to the negative battery bus-bar 143. Thus the potential of each of the repeater bus-bars has been lowered by an amount equal to the voltage of cells 131 and 132 and cells 135 and 136, respectively. When contact 13 of relay 1.1 is again closed followed by the release of relays 4, 10 and 11, as hereinbefore described, the circuit of the polarized relay 1 is again completed. However, the potential connected to the terminal of the polarized relay by way of bus-bar 142 has now been lowered so that the difference in potential on the terminals of the polarized relay is less than 5 volts and the relay will not operate.

From the foregoing detailed explanation it will be seen that the potential of each of the bus-bars 142- and 143 is automatically raised or lowered so that the potentials connected to the repeater relays such as 60 and 127 in the sub-exchange areat all times substantially the same as the potentials connected to the automatic equipment in the main exchange. Each of the auxiliary cells, such as 131, may consist of three dry cells in order that the potentials of the auxiliary or repeater bus-bars may be changed in steps of 5 volts since the polarized relay 1 is adjusted to operate on an approximate difference in potential of 5 volts. Thus the equipment shown in the drawings provides for a variation of 20 volts, either way, in the ground potential of the sub-exchange from that of the thought to be new and is desired to have protected by Letters Patent will be pointed out in the appended claims.

What is claimed is:

1. In a telephone system, two exchanges, a

test circuit connecting said exchanges and normally connected to ground in each exchange, a relay 1n said test c rcuit in the second exchange operative when the difi'erence in ground potential between said exchanges exceeds a predetermined amount, and means controlled by said relay for making a unit voltage adjustment in said second exchange.

2. In'a telephone system, two exchanges, a test circuit connecting said exchanges and normally connected to ground in each exchange, a relay in said test circuit responsive to differences in ground potential above a predetermined amount, means responsive to the operation of said relay for opening said test circuit and for making a unit voltage adjustment, and means for thereafter reclosmg said test circuit to determine if said adjustment is suflicient.

3. In a telephone system, two exchanges, a test circuit connecting said exchanges and normally connected to ground in each exchange, means for opening said test circuit and for making a unit voltage adjustment in the second exchange when the difference in ground potential exceeds a predetermined amount, and means for reclosing said test circuit after said adjustment has been made to determine if the adjustment is sufiicient.

4. In a telephone system, two exchanges, a test circuit between said exchanges normally connected to ground in each exchange, and a step-by-step switch operated over said test circuit when the difierence in ground potential between said exchanges exceeds a predetermined amount.

5. In a telephone system, two exchanges, a test circuit between said exchanges normally connected to ground in each exchange, a two-direction stepping switch operated over said circuit responsive to differences in ground potential between said exchanges, and means responsive to the operation of said switch for either raising or lowering the potential of the ground connection in one exchange depending upon the direction in which said switch was operated.

6. In a telephone system, two exchanges, a test circuit between said exchanges normally connected to -.ground in each exchange, a polarizedrelay in said test circuitoperated to eitherone of two positions responsive to a difference in ground potentia-l between said exchange and dependent upon the direction of said potential difference, and a two-direction stepping switch controlled by said relay and operated in one or the other of its movements dependent upon the positionto which said relay was operated.

7. In a telephonesystem, two exchanges,

a battery ineach exchange, each battery having its terminals connected-to apair of busbars, one or" said bus-bars in each exchange being connected to ground, a compensating circuit comprising a single conductorconnected tothe grounded bus-bar in each exchange, and means controlled oversaid circuit for raising and lowerlng the potentials I of'the bus-bars in one exchange'to agree at alltimes with the potentials of the corresponding bus-b ars in the other exchange.

8. Ina telephonesystem, two exchanges, a

battery in each exchange, a, pair of main'busbars in each exchange connected tothe'two poles or" the respective batteries, one of said bus-bars in each exchange being connected to ground, a pair of auxiliary'bus-bars-in the second exchange normally con'nectedto the main bus-bars in'that exchange, a circuit conrccting the main ground busb'arin'the first exchange and the auxiliary gro'undbus-bar in the second exchange, and means controlled over said circuit'for maintaining the auxiliary bus-bars of the second exchange at the same potentials asthe main bus-bars of the first exchange.

QQ'In a telephone system,'two exchanges each including abattery and apair of main "bus-bars connecte d'to the terminals ofthe 'battery, one bus-bar in each exchange being connected to ground, a pair of auxiliary busbars in the second exchange norm-ally connected to the main 'bus-bars in that exchange, and means for automatically maintainlng said auxiliary bus-bars at the samepotentials as the main bus-barsin'the first exchange 1rrespective'of the potentialsof the main bus- -bars in the second exchange.

10. In a telephone system, two exchanges each including a battery having one terminal 'connected'tO :ground, aipair of bus-bars in it-he second exchange-normally connected to ber of said cellsin 'the connections-between "the terminals of the exchange battery and-said bus-bars to maintain-the bus bars at the same potentials as the-terminals of the battery in the first'exchange.

11. In combination, 'two batteries each having one terminalconnected to -.ground,-a

pair of bus-bars normally-connectedto the two terminals, respectively, of the second battery, and means;operative'whenthe difference in potential between said two ground connections exceeds a predetermined amount for adjusting thepotentials at saidbus-bars to agree With'the potentials of 'the terminals of the'firstbattery.

'l2. In combination, two batteries each having one'terminal connected to ground,'a

pair of'bus-bars normally connected to the two terminals;respectively, of thesecond battery, a circuit connecting'the grounded "terminal of the first battery and the'bus barnormally connected to'the grounded termin'al'of the second battery, and means controlled over said circuit for maintaining said bus bars at the same potei'itials asthe terminals of the first battery.

"13. In combination, two batteries each having one terminal connected to-ground,'a

1 pair of 'bus' bars normally connected to "the twoterminzllsof the second battery, respectively, a polarized 'relay, 'a circuit for said relay includingthe grounded terminal of the first battery an'dthe'normally grounded busbar,a' plurality 'of-auxiliary cells, and means controlled by said polarized relay for inserting a predeterminednumber of said cells'in the connections between saidsecond battery andsaid 'bus b'ars'in order "to maintain said bus-bars at the samefp'otentials as'the terminals of said first battery.

1%. In combination, two batteries each having one ipoleconn'ectedto ground, a pair of bus bars normally connected to the two terminalsof the second battery, respectively, a polarized relay connected between the grounded pole of the first battery and'the normally grounded bus-bar, saidrelay being adjusted to operate whenthe difierence in ground potential exceeds a predetermined amount,a swltchh'aving a'forward and a backward movement, means controlled by saidrelay for"operating said switch in either OfltS'Il'lOVGIIlGIltS depending'upon the direc- 'tion of potential difference atthe'terminals of said "relay, a pluralityof auxiliary cells,

and means responsive to the operation of said switch for inserting'a. number ofcells 1n the connections between saldbus-bars and "the terminals of said secondbattery toeither raise or lower the potentials of-said bus-bars depending upon the direction'in which'said switch was operated.

15. The method of voltage adjustment whichconsists-inmaking a continuous test for differences in ground potential, in making a unit adjustment in voltagewhenthe difference --1n ground potential exceeds a predetermined amount, in stopping the test'1 

